A man who put an advert on a website seeking underage girls for sex has been jailed after the website's administrator informed authorities.

Orchard worker Michael Worsnop, 29, used the nickname "hellyeah4young" in the advert posted in July 2012.

The advert contained images of two partly naked underage girls in sexualised poses.

According to a summary of facts the unnamed website administrator referred the advert to the Department of Internal Affairs.

When Worsnop was contacted in March last year he admitted placing the advert and said he had an interest in girls aged 12 and older.

He was found to have 154 images of underage children on his phone.

In Napier District Court yesterday, Judge Bridget Mackintosh sentenced Worsnop to 17 months' jail on three charges of possessing objectionable material and one charge of making objectionable material.

He was also convicted of three charges of wilful trespass and one of using insulting language in relation to separate offending.

Department community safety manager Steve O'Brien said that Worsnop's erratic employment and regular switching of accommodation made him difficult to contact but inspectors eventually found him living in a caravan in an orchard in the Meeanee-Awatoto Rd, Hawke's Bay.

O'Brien said phone records were particularly useful in this case. "It helps us enormously when telephone companies help us track online offenders," he said.

"Holding these people to account is a team effort often involving local or international agencies.

"And we have the expertise to gather the evidence, despite best efforts to conceal offending, and bring offenders before the court."

When freed from jail, Worsnop will face special conditions for six months. These will include a ban on alcohol and drugs and restrictions on his access to the internet.